Switch to fourmolu source code formatter

[vaclavsvejcar]

Unfortunately currently used brittany seems to be dead and it's more and more trouble to use it for formatting headroom code base. Aim of this meta-task is to consider other source code formatter and eventually reformat the codebase in one run.

• Brittany fails to format code with Haddock comments in module export list lspitzner/brittany#365
• Brittany fails to format code with type families lspitzner/brittany#340